Add dev branch setup

[Wunka]

This doesn't mean we should go complete one branch per version setup. But I think it's something which would be really good to have. This way the wiki can already be updated for an unreleased version of Cubyz.

The main branch should be on a regular basis be merged into dev to discover merge conflicts and keep it up to date.

And on a new release of Cubyz the dev branch should be merged into main.

It can be up to discussion if the dev branch should also be published on the github pages. (In my opinion it would be nice)

[Argmaster]

We can just keep main as the development branch and add "stable" branch, since realistically most people don't are about it.

For the sake of discussion let's keep the nomenclature as in the issue.

It is not enough to merge dev into main on release - you will have to backport stuff like typo fixes with cherry-picks, so histories of both branches will diverge. I suspect that sooner or later, as the changes overlap, you will run into severe conflicts.
The safe way to do this is to delete stable branch on release and create it again from develop. That is exactly equivalent to just making a new release/x.y.z branch and not supporting the older release branches. The benefit is that you can always restore that release state and extend it if you need to do so.

[Wunka]

I was meaning to have:

• dev: only merge into when the change to the wiki affect an unreleased Cubyz version (e.g permission Groups)
• main: merge typos and general text that affects the current release of cubyz

But you have probably a better idea (and experience) of how to structure this.

[Argmaster]

This is very optimistic, but you can't expect contributors, especially external, to figure out our personalized development model.
It is not standard thing to do and we will annoy people if you ask them to change base branch on every second PR.
You could have one contributor facing branch and cherry-picks everything, but thats clearly a lot of extra work.